This invention relates to radioisotope cameras, and particularly to an improved, position-sensitive readout system for a multi-crystal camera.
Radioisotope cameras have considerable important uses in medical diagnosis and research. They have a considerable advantage over the use of X-ray techniques in that they are sensitive to minute amounts of certain radioactive tracer compounds. In use, a small quantity of a gamma-ray emitting radioactive substance is injected into a patient. The choice of radioactive isotope depends on its half-life, activity, dose rate, and many other factors. Certain isotopes are highly specific, and tend to concentrate in certain organs of the body. This selective accumulation permits visualization the biological function of almost every organ of the human body. Although the resolution of radioisotope cameras is not equal to that obtained in X-ray radiographs, the size, shape, position and function of the organs can be determined, and often lesions can be located in them.
In general, a radioisotope camera consists of a pinhole or multi-channel collimator and a position-sensitive gamma-ray detector.
A very commonly used radioisotope camera is the gamma camera invented by Hal O. Anger at the Donner Laboratory, University of California, Berkeley. H. O. Anger: "Survey of Radioisotope Cameras," ISA Transactions, 5:311-334, 1966. It consists of a single NaI (Tl) crystal, typically 12 inches in diameter and 0.5 inches thick, and a number of photomultiplier tubes which are optically coupled to the crystal. A gamma-ray, from an emitting source, which strikes the crystal will cause an optical scintillation flash that is detected by the phototubes. A network of capacitors connects the output of all phototubes to 4 amplifiers where outputs are combined in a special circuit that determines the x and y spatial coordinates of the center of intensity of the scintillation flash. The locations of a sufficient scintillation events will thus enable a visual representation of the gamma-ray emitting source to be obtained. A significant disadvantage of this design is that the entire crystal is used for the detection of each gamma-ray, resulting in a maximum detectable event rate of below 50,000 events per second.
This limitation was largely overcome by the development of a multi-crystal gamma camera by Merrill A. Bender and Monte Blau, described in M. A. Bender, M. Blau: "The Autofluoroscope," Nucleonics, Vol. 21, No. 10, October, 1963, pp. 52-56. This camera uses an N.times.M array of closely packed 10 mm.times.10 mm NaI (Tl) scintillation crystals, each coupled to two of N+M remotely-positioned phototubes by a pair of long twisting light pipes. One of the phototubes determines the row of a scintillation crystal and the other phototube determines the column. With this design, useful event rates of 200,000/second have been achieved. However, this design has a number of significant problems such as the very poor light transfer from the scintillation crystals to the remote phototubes and the difficulty of fabricating and assembling the 2.times.M.times.N lightpipes for a large camera.
The light transfer from a mosaic of scintillation crystals to the phototubes could, of course, be greatly increased by disposing the phototubes immediately adjacent to the crystals. Although the scintillator crystals can be made as small as desired, the phototubes cannot. As a consequence, the spatial resolution would be determined by the size of the closely packed phototubes. Much research, in many countries, has been done to make phototubes as small as possible to improve resolving power, but the smallest phototube presently available is 10 mm in diameter. Such size is a severe limitation on resolution. With the Anger camera principle, resolution of 3-4 mm has been achieved. However, the deadtime from the afterglow in the common large crystal is much worse than that of a mosaic of crystals.
Further, phototubes are relatively expensive and the individual coupling of phototubes to a mosaic of scintillation crystals would be very costly.